Many years ago, Analog science fiction magazine detailed how to build a crude atomic bomb. The method involved depositing one subcritical mass at the bottom of a small diameter well shaft below your (not-so) safe house’s floor and then using an explosive charge to accelerate the other subcritical half into the former in order to precipitate the chain reaction.
As with any accurate article on this topic it was noted that obtaining the fissile material was by far the most difficult aspect of the task. The most ready source of the uranium would come from hijacking a truckload of unrefined reactor fuel. This immediately posed the extremely complex problem of refining said fuel into metallic form.
The final upshot of the article noted that it was much easier to commandeer a nuclear reactor facility and intentionally melt it down. The major drawback of this strategy was that it would not meet any tactical requirement, in that power generating reactors are not sited near critical military facilities. They are more often adjacent to population centers which would not serve war time goals.
A small amount of rethinking is necessary in this day of terrorist activities, but I am confident this is the reason that security at reactor locations has been stepped up significantly in recent months. Still, it is worth noting that whenever special forces have conducted exercises aimed at testing the feasibility of taking over a nuclear power plant, they have always been able to do so with ease.